This invention relates to a drive control apparatus of an electromagnetic feeder used in a combinatorial weighing system or the like. More particularly, the invention relates to an electromagnetic feeder drive control apparatus for controlling the amplitude of oscillation of an electromagnetic feeder in precise and correct fashion, and which is adapted to electrically isolate power supply circuitry and a signal processing circuit through use of a photocoupler in order to effect amplitude control stably by means of digital control.
A combinatorial weighing system in practical use operates by supplying a plurality of weighing machines with small quantities of articles to be weighed, weighing the articles supplied to each machine, applying a signal indicative of the weight of the articles in each weighing machine to a computation control unit constituted by a microcomputer, having the computation control unit perform a combinatorial computation based on a predetermined pattern, selecting a combination, which satisfies prescribed conditions, as a discharge combination, and discharging the articles from the weighing machines corresponding to the selected combination. An electromagnetic feeder is used to supply each weighing machine with the articles to be weighed.
An example of a conventional drive control apparatus for an electromagnetic feeder of this kind is as illustrated in FIG. 1. The electromagnetic feeder, designated at numeral 2, is connected to an AC power supply 1 through a thyristor (SCR) 3. The oscillation amplitude of the electromagnetic feeder 2 is controlled in the following manner. Specifically, a digital signal is produced as an output by a drive control circuit 4 incorporated within an electronic computation control unit constituted by a microcomputer. The trigger pulse commanding signal is converted into an analog signal by a D/A converter circuit 5. The analog signal causes a light-emitting diode 6 to emit light, which irradiates a light-receiving element 7. The latter converts the light signal into an electrical signal which is applied to a CR time constant circuit comprising a variable resistor 8 and a capacitor 9. When the capacitor 9 is charged to a voltage in excess of a prescribed value, a trigger circuit 10 is actuated and the capacitor 9 is discharged, thereby applying a trigger pulse to the gate of the SCR 3 to drive the SCR 3 into conduction.
More specifically, the conventional drive control apparatus for the electromagnetic feeder 2 controls the amplitude of a current applied to the electromagnetic feeder 2 by applying a trigger pulse to the gate electrode of the SCR 3 upon expiration of a time period determined by the time constant of the CR time constant circuit, the time period beginning at the instant the power supply voltage crosses zero.
Thus, in conventional control of an electromagnetic feeder, the SCR trigger pulse commanding signal from the drive control circuit 4 is applied to the gate electrode of the SCR 3 through the CR time constant circuit. Therefore, phase control cannot be carried out while the CR time constant circuit is being charged. In addition, due to variance in the characteristics of the CR elemenrs, there are cases where correct phase control of the trigger pulse cannot be performed. Another disadvantage is a complex circuit construction.